Certain conventional antenna systems that adapt radiation patterns to meet conditions in a wireless communication system are referred to as "smart antennas" or "adaptive antenna arrays". Typically, an adaptive antenna array uses a group of receivers to determine a suitable radiation pattern that maximizes the received signal strength of an uplink transmission. If multiple receivers are mounted with the antenna system on a tower-top, replacement and repair of the antenna system becomes difficult and costly. Further, many conventional adaptive antennas only provide increased uplink receiver performance without any corresponding benefit for downlink transmission. Thus, a need exists for a reliable antenna system that provides enhanced performance for both uplink and downlink signals in a wireless communications system.
Conventional adaptive arrays for uplink reception may be used in conjunction with a separate switched, fixed beam system for downlink transmission. For example, certain antenna systems use a "sub-sectored" approach for uplink reception as a mechanism for making the correct switching choice for a related downlink transmission. However, using multiple antenna arrays may exceed the limits of available mounting space. Moreover, using multiple antennas may increase transmission line, duplexer, cable, and other hardware requirements.
To simplify antenna design, some adaptive reception antennas balance their reception improvement by merely increasing the base station's downlink power for transmission. The power increase not only places a burden on the base station's amplifier system for range extension, but also does nothing to improve the signal-to-interference (S/I) ratio. An improvement in the S/I ratio may afford an increase in the traffic capacity or frequency-reuse density. Thus, a need exists for fully adaptive pattern shaping of downlink transmission to enhance the signal-to-interference ratio of the downlink signal.